Connected: An Internet Encyclopedia
RFC 826 - Packet format

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### Packet format

To communicate mappings from pairs to 48.bit Ethernet addresses, a packet format that embodies the Address Resolution protocol is needed. The format of the packet follows.

        Ethernet transmission layer (not necessarily accessible to
    	 the user):
    	48.bit: Ethernet address of destination
    	48.bit: Ethernet address of sender
    	16.bit: Protocol type = ether_type$ADDRESS_RESOLUTION
        Ethernet packet data:
    	16.bit: (ar$hrd) Hardware address space (e.g., Ethernet,
    			 Packet Radio Net.)
    	16.bit: (ar$pro) Protocol address space.  For Ethernet
    			 hardware, this is from the set of type
    			 fields ether_typ$.
    	 8.bit: (ar$hln) byte length of each hardware address
    	 8.bit: (ar$pln) byte length of each protocol address
    	16.bit: (ar$op)  opcode (ares_op$REQUEST | ares_op$REPLY)
    	nbytes: (ar$sha) Hardware address of sender of this
    			 packet, n from the ar$hln field.
    	mbytes: (ar$spa) Protocol address of sender of this
    			 packet, m from the ar$pln field.
    	nbytes: (ar$tha) Hardware address of target of this
    			 packet (if known).
    	mbytes: (ar$tpa) Protocol address of target.

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Connected: An Internet Encyclopedia
RFC 826 - Packet format